1. Field of the Invention
This invention relates to an automatic transmission for a bicycle. More particularly, this invention relates to a multi-speed bicycle employing a multi-stage sprocket assembly, wherein operation of the derailer thereof is automatically controlled by centrifugal force so that the driving chain is automatically shifted to higher gears as the speed of the bicycle increases.
2. Description of the Background Art
Presently there exists many types of bicycle transmissions. The earliest type of a bicycle transmission consisted of a rear wheel having a three-speed hub whose internal gearing was manually shifted by means of a selector cable extending from a selector lever conveniently positioned on the handle bars to a movable selector axle within the rear hub. During cycling, the three-speed bicycle could be easily shifted into low, medium and high gears by moving the selector lever to a low, medium or high position, respectively.
In subsequent years, improvements were made to the conventional three-speed bicycles to make the gear shifting in the three-speed hub fully automatic depending upon the speed of the bicycle. Thus, starting from rest, the automatic transmission of the three-speed hub would be initially set to the low gear and, upon acceleration of the bicycle to higher speeds, would automatically shift to middle gear and then to high gear. This automatic shifting therefore obviated the need for the bicyclist to manually change gears via a manual selector lever.
U.S. Pat. Nos. 3,494,227, 3,492,892, 3,492,893, 3,081,641, 3,513,726, 3,520,214, 3,546,970, 3,546,971, 3,552,233, 3,592,081, 3,600,974, 3,603,178, 3,608,398, 3,648,547, 3,659,688, 3,661,034, 3,696,690, 3,701,292, 3,830,521 and 4,229,997 disclose various embodiments of automatic three-speed rear hubs. In most of these prior patents, the automatic shifting feature is accomplished through the use of centrifugal weights positioned within the hub and centrifugally operated to shift gears upon increased rotational speed of the hub corresponding to increased speed of the bicycle.
In more recent years, the use of three-speed bicycles have been largely supplanted by ten-speed (or more) bicycles. More particularly, a conventional ten-speed bicycle comprises a pair of drive sprockets of different diameters positioned at the axis of rotation of the bicycle pedals and a multi-stage sprocket assembly consisting of five drive sprockets of different diameters positioned at the axis of rotation of the hub of the rear wheel of the bicycle. The pedal drive sprockets and the rear wheel drive sprockets are operatively interconnected by means of a drive chain. Mechanisms known as "derailers" are provided for derailing the drive chain from one sprocket to another of the two pedal sprockets and from one sprocket to another one of the five rear sprockets thereby changing the gear ratio. A control lever is connected to each derailer by means of a control cable. Thus, selected movement of the control levers controlling the respective derailers allows the bicyclist to select any one of ten possible gear ratios thereby resulting in "ten-speeds". It is also noted that a tension wheel is provided to take up slack in the drive chain thereby making sure that the drive chain remains taut at any gear speed. U.S. Pat. Nos. 4,895,553, 3,661,021, 4,277,077, 4,580,997, and 4,612,004 disclose various gearing mechanisms employed in a conventional ten-speed bicycle. It is noted that even more recently, twenty-one-speed bicycles have been developed which comprise three pedal sprockets and seven rear sprockets, thereby providing twenty-one possible speed combinations.
Analogous to the development of three-speed bicycle hubs, most recent developments of ten-speed bicycles have consisted of employing centrifugal weights which function to automatically control the gear shifting depending upon the speed of the bicycle. For example, in U.S. Pat. No. 3,513,725, an automatic speed change mechanism is disclosed for a ten-speed bicycle wherein the automatic shifting feature is accomplished with the use of centrifugal weights positioned within the multi-stage bracket assembly. In U.S. Pat. No. 4,836,046, an automatic bicycle transmission is disclosed which comprises a sprocket wheel pedal assembly including a reference wheel that is rotated by the pedals of the bicycle. A pair of diametrically opposed chain gear sliders and opposed chain gliders are provided for radially moving along radial slots of the sprocket wheel assembly in response of input torque applied at the pedals, thereby changing the effective diameter of the front sprocket and causing speed changes. In U.S. Pat. No. 4,743,041, an automatically variable gear ratio drive mechanism for a bicycle is disclosed which comprises a pulley whose diameter varies by means of a centrifugal weight in such a manner that the effective diameter of the pulley decreases as the bicycle's speed is increased, thereby functioning to change the effective gearing ratio.
Similarly, U.S. Pat. No. 4,701,152 discloses an automatic transmission for a multi-speed bicycle having a multi-stage rear sprocket assembly. The automatic gear shifting is accomplished by means of a plurality of centrifugal weights which are slideably disposed on the spokes of the rear wheel. The centrifugal weights are operatively connected by means of cables to extensible pistons rigidly connected to the rear hub of the bicycle. The extensible rods of the pistons are connected about the periphery of a ring-shaped support which, upon extension of the piston rods, extends outwardly to be in the respective planes of the sprockets of the multi-stage sprocket assembly. In its preferred embodiment shown in FIGS. 11 and 12 thereof, a derailer is operatively connected to the peripheral edge of the ring-shaped support in such a manner that the derailer is forced into alignment with the selected sprocket by the ring-shaped support as determined by the centrifugal force imparted to the pistons by means of the centrifugal weights. Unfortunately, it has been found that the centrifugal weights sliding on their respective spokes results in undesired downshifting of the gears the moment the bicyclist resumes pedaling after coasting. Furthermore, some bicyclists have found it desirable to be able to select an upper gear that the bicyclist feels comfortable using, even though the desired gear may not be the highest gear of the multi-speed bicycle. The automatic transmission disclosed in U.S. Pat. No. 4,701,152 fails to disclose any means for limiting the movement of its centrifugal weights so as to limit the corresponding movement of its ring-shaped support and its derailer selecting the highest sprocket, corresponding to the highest gear desired. In U.S. Pat. No. 3,540,309, similar concepts are employed for centrifugally causing gear changing of the pedal change sprocket.
Therefore, it is an object of this invention to provide an apparatus which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the advancement of automatic transmissions for bicycles.
Another object of this invention is to provide an automatic transmission for a bicycle having a multi-stage rear sprocket assembly and derailer, wherein the automatic derailing of the drive chain is accomplished by means of centrifugal weights operatively connected to the derailer to move the derailer in response to increased speed of the bicycle.
Another object of this invention is to provide an automatic transmission for a bicycle having a multi-stage rear sprocket assembly and derailer whose operation is controlled by centrifugal weights, wherein the centrifugal weights allow the bicyclist to resume pedaling after coasting in the same gear that existed prior to coasting.
Another object of this invention is to provide an automatic transmission for a bicycle having a multi-stage rear sprocket assembly and derailer controlled by centrifugal weights, wherein means are provided for selecting a range of operation of the centrifugal weights so as to allow the bicyclist to select a shifting range employing less than all of the possible gearing combinations.
Another object of this invention is to provide an automatic transmission for a bicycle having a multi-stage rear sprocket assembly and derailer controlled by centrifugal weights, and further including means for limiting the highest gear into which the centrifugal weights moves the derailer into so as to permit the bicyclist to select an upper limited gear which is less than the highest possible gear.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.